This invention relates to dimensional checking apparatus and particularly to apparatus for measuring the dimensions of nuclear fuel pellets.
In many designs of nuclear reactors, the reactor vessel has an inlet and outlet for circulation of a coolant in heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound, is stacked within the matallic sheath. During reactor operation, the nuclear fuel pellets fission releasing fission products such as fission gas while generating heat in a manner well known in the art.
There are many known methods for manufacturing the nuclear fuel pellets used in nuclear reactors. Most of these methods generally consist of cold pressing a powder which may be an oxide of fissionable material such as uranium dioxide to form dense compacts. These dense compacts are generally referred to as green pellets. The green pellets are then sintered in a non-oxidizing atmosphere to produce a sintered pellet which may have slight irregularities on its surface. The sintered pellet may then be ground to remove those irregularities thereby forming a right cylindrical pellet. This finished pellet is then stacked within the metallic sheath to form the fuel element that may be used in a nuclear reactor.
A commonly known method for producing the nuclear fuel pellets is described in U.S. Pat No. 2,991,601 to J. Glatter et al, issued July 11, 1961. In this process, hydrogen reduction of uranium trioxide is employed to produce uranium dioxide powder. As received from commercial manufacturers, this uranium dioxide is not free flowing and is, therefore, not adaptable for use in automatic machinery for the production of the green pellets. In order to produce a free flowing powder, the uranium dioxide powder is mixed with a suitable binder such as aluminum stearate and water to form a wet granulate. The wet granulate is then forced through a screen and dried, after which it is dry-screened thereby separating the larger particles from the smaller particles. The water may be substantially removed in the later sintering process while the aluminum stearate will remain and act as a lubricant in the compacting process. Once the uranium dioxide powder has thus been converted into a free flowing granulate, the granulate is then compacted into green pellets in a cold pressing operation. The compacting process comprises flowing the granulate into a die and cold pressing the granulate in the die into substantially cylindrical green pellets. The green pellets may then be heat treated, sintered and ground to form the finished pellet for use in nuclear fuel elements.
With the demand for nuclear fuel increasing it has become a commercial necessity to be able to mass produce the green pellets. The known procedures for mass producing green pellets have generally involved open area access to the apparatus and green pellets. This type of procedure is acceptable when the nuclear fuel used in the pellet is a non-irradiated uranium compound because such nuclear fuel does not pose serious radiological problems for working personnel. However, when the nuclear fuel employed is plutonium or a reprocessed uranium compound, increased safeguards are necessary to insure that working personnel do not become overexposed.
One step in the manufacture of the nuclear fuel pellet is the inspection of the green pellet to ascertain if the pellet dimensions are within acceptable limits. Of particular importance is the green pellet density. The conventional method of checking green pellet density is for the pellet press operator to remove a pellet by hand from the pellet stream leaving the pellet press, measure the pellet length by hand micrometer, determine the weight of the pellet by placing it on a gram balance, and then using a chart with length-weight coordinates determine the green pellet density. While this manual operation is acceptable with a less toxic fuel, a more toxic fuel requires that this procedure be performed in a glove box environment to thereby protect the operator from radiation exposure. However, a glove box type procedure is not adequate when it is desired to mass produce the green pellets. It is, therefore, desirable to eliminate operator handling from the green pellet dimensional check by mechanizing the sequence of operations thus permitting the operator to monitor the operation remotely.